This invention relates to a method of and apparatus for reducing the duration of the thermal stabilisation phase of a liquefied gas converter and more particularly to such a method of and apparatus for converting liquefied oxygen to breathable oxygen gas.
Oxygen converters are well known and one such oxygen converter for use by an air crew in an aircraft is described in our previous patent GB 1303046. In the arrangement described, there is a liquefied oxygen store comprising an insulated dewar. When the dewar is filled with or topped up with liquefied oxygen, liquefied oxygen is fed via a valve, along a feed line, which includes an uninsulated container and a heat exchanger.
When the converter is in use, liquefied oxygen passes back along the feed line into the heat exchanger and uninsulated container where the liquefied oxygen gains heat and is converted into oxygen gas.
It will be appreciated by those skilled in the art that immediately after filling, only the surface layer of the liquefied oxygen is at a temperature consistent with the desired system pressure. As the liquefied oxygen has a low thermal conductivity, the subsequent thermal stabilisation phase when the temperature of the liquefied oxygen in the converter stabilises, can be of considerable duration, typically of about 24 hours in some known systems.
Where the oxygen converter is installed in an aircraft, during the thermal stabilisation phase the aircraft preferably is not used, as oxygen at an appropriate pressure for supply to the breathing system cannot be guaranteed until the thermal equilibrium of the liquefied oxygen bulk in the dewar is established. This is because disturbance of the liquefied oxygen, by vibration say during aircraft take off, tends to cause mixing of the surface layer and the remainder of the bulk. Thus the temperature of the surface layer would be reduced resulting in a loss of pressure.
Various methods have been utilised to reduce the duration of the thermal stabilisation phase. All of these essentially involve increasing the temperature of the liquefied bulk towards the surface layer temperature. For example, in our previous patent GB 1303046, during filling, as the liquefied oxygen passes through the uninsulated container and the heat exchanger of the liquefied oxygen converter, heat energy is gained.
In each existing design of liquefied oxygen converter a unique approach has to be taken to reducing the duration of the thermal stabilisation phase.
It is one object of the present invention to provide a method of and apparatus for reducing the duration of the thermal stabilisation phase of a liquefied gas converter which is more generally applicable.
According to a first aspect of the invention we provide a method of reducing the duration of the thermal stabilisation phase of a liquefied gas converter characterised in that the method comprises providing in a feed line between a source of liquefied gas and the liquefied gas converter, an uninsulated flow region through at least a portion of which the liquefied gas passes during filling.
Thus the duration of the thermal stabilisation phase of a liquefied gas converter can substantially be reduced without any adaptation of the existing liquefied gas converter. Thus the method is applicable irrespective of liquefied gas converter design and may be performed readily on existing systems.
Thus the method may include interrupting an existing feed line and connecting an inlet of the uninsulated flow region to a part of the feed line which extends to the liquefied gas store, and connecting an outlet of the uninsulated flow region to a pair of the feed line which extends to the liquefied gas converter. The inlet and outlet of the uninsulated flow region may be separate or combined.
Preferably the method includes locating the uninsulated flow region in an environment which is at ambient temperature. Thus the liquefied gas gains heat energy from the ambient environment as it flows through the uninsulated flow region.
According to a second aspect of the invention we provide an apparatus for reducing the duration of the thermal stabilisation phase of a liquefied gas converter having a liquefied gas feed line between a source of liquefied gas and the liquefied gas converter, and a product gas outlet characterised in that the apparatus comprises an uninsulated flow region located in the feed line through which at least a portion of the liquefied oxygen passes during filling of a liquefied gas store of the liquefied gas converter.
Thus the apparatus of the second aspect of the invention may be provided utilising an existing liquefied gas converter. The liquefied gas converter and the uninsulated flow region of the apparatus may be installed together or the uninsulated flow region may be retrofitted to an existing gas supply system.
The uninsulated flow region may have a capacity of between 5% and 15% of the volume of the liquefied gas store of the oxygen converter and more particularly may have a capacity of between 7% and 10% of the volume of the liquefied gas store of the oxygen converter. However it will be appreciated that in order to achieve a desired thermal stabilisation phase duration the preferred volume of the uninsulated flow region will depend on many other factors, including amongst others, the rate of filling, the temperature of the environment in which the uninsulated flow region is located and the effectiveness of insulation of the remainder of the feed line.
The uninsulated flow region may comprise a simple container through which at least a potion of the liquefied gas flows during filling, or particularly where there is a restriction of available space, for example in an aircraft, the uninsulated flow region may comprise an inlet and an outlet and a flow passage or passages between the inlet and the outlet of a length, or combined length which is substantially greater than the distance from the inlet to the outlet.
According to a third aspect of the invention we provide a method of adapting a gas supply system comprising a liquefied gas converter, a feed line to the liquefied gas converter for replenishing a liquefied gas store of the liquefied gas converter, and a product gas outlet for product gas produced by the liquefied gas converter, the method comprising installing in the feed line an uninsulated flow region through which at least a portion of the liquefied gas flows during filling of the liquefied gas store, whereby the duration of the thermal stabilisation phase of the liquefied gas converter subsequent to filling, is reduced.
The invention will now be described with reference to the accompanying drawings which is a diagrammatic representation of an apparatus of the second aspect of the invention.